Abstract

There is an increasingly body of evidence, obtained both in vitro and in vivo, showing that exogenous opioids have a variety of effects on cells of the immune system. The consequence is that opiates at pharmacological concentrations suppress cell-mediated immunity, as reflected by depressed T-dependent antibody production by B lymphocytes, altered T lymphocyte functions such as proliferation, delayed-type hypersensitivity, graft-versus-host responses and decreased cytotoxic NK cell activity. The macrophage/monocyte oxidative burst and phagocytosis are also impaired, effects probably mediated by various opioid receptor types as they are blocked or reversed by naloxone, an opioid antagonist. Other possible mechanisms of interaction remain to be elucidated: exogenous opioids can act on neurons of the central nervous system, thereby activating the neuroendocrine system with a subsequent increase in serum glucocorticoid levels. Another potential link between the central nervous system and lymphoid tissue is the sympathetic nervous system, via which opioid-induced activation could result in noradrenergic inhibition of the immune system. The clinical consequences of these suppressive effects on the immune system are seen in the striking increase in the incidence of infections in intravenous opioid addicts. The advent of AIDS and the identification of intravenous drug abusers as a critical risk group have propelled interest in this area. Data obtained both in vitro and in vivo with various experimental models shows that morphine increases susceptibility to bacterial and viral infections, the latter effect possibly being related to a depressive effect of opioids on gamma-interferon levels. The dosage and time of administration strongly influence the results: it appears that chronic opioid treatment in vivo induces a state of immune tolerance, with normal resistance to viral infections, whereas short or single administration has a detrimental effect. In the former context, other factors such as a morphine-induced increase in CD4+ cell numbers may tend to enhance the infectivity of HIV-infected subjects.